Lower minimum required Sphinx version

Lower the minimum required Sphinx version from 4.0 to 3.0 again.

Note that Sphinx version 4.1.0 does not render the html logo and favicon
due to a bug (sphinx-doc/sphinx#9438).
However, this version is not explicitly excluded from the requirements
file, because pip tries to avoid to install this version automatically.

docs/requirements.txt

@@ -1,8 +1,9 @@
-# Date: 2021/06/26
 # Requirements to build MDTools' documentation.
-# These requirements apply additional to the install requirements of in
-# the root level of MDTools.
+#
+# These requirements apply additionally to the install requirements of
+# MDTools as listed in the requirements file in the root directory of
+# the project.
 
-sphinx >=4.0, <5.0
+sphinx >=3.0, <5.0
 sphinx-rtd-theme >=0.5, <1.0
 readthedocs-sphinx-search >=0.1, <1.0

docs/source/conf.py

@@ -102,7 +102,7 @@
 source_suffix = {".rst": "restructuredtext"}
 
 # The document that contains the root toctree directive.
-root_doc = "index"
+master_doc = "index"
 
 # A list of glob-style patterns that should be excluded when looking for
 # source files.  They are matched against the source file names relative
@@ -200,7 +200,7 @@
 default_role = None
 
 # If your documentation needs a minimal Sphinx version, state it here.
-needs_sphinx = "4.0"
+needs_sphinx = "3.0"
 
 # If true, Sphinx will warn about all references where the target cannot
 # be found.
@@ -360,8 +360,8 @@
 
 # latex_documents determines how to group the document tree into LaTeX
 # source files.
-_startdocname = root_doc
 _targetname = htmlhelp_basename + ".tex"
+_startdocname = master_doc
 _title = "MDTools Documentation"
 _author = r" \and ".join(author.split(", "))
 _theme = latex_theme